The present invention relates to improvements in forming a stable aqueous gel, and more particularly to an improved process for preparing a strong, salt-tolerant and heat-stable aqueous gel useful in improving the water-oil ratio in oil production or useful as an agent for cutting off water used in civil engineering and construction works.
In oil production and underground engineering works, leakage of underground water often causes problems. A technique of blocking the passage of underground water using a gel has been adopted for preventing the leakage of underground water.
The gel is usually formed by gelling a polymer dissolved in underground water containing salts in high concentrations with a gelling agent. The gel is required to have an adequate gelation time so that it gels at a desired place, as well as high gel strength and a good long term stability. For this purpose, water-soluble polymer such as acrylamide polymers, mixtures thereof with biopolymers such as xanthan gum, and carboxymethyl cellulose have been hitherto employed in the form of an aqueous solution.
Polyvalent metal ions such as Al.sup.3+, Cr.sup.3+, Ca.sup.2+ and Mg.sup.2+ are employed for the gelation, and among them, the gelation using Cr.sup.3+ or Al.sup.3+ has been practiced.
In case of the gelation with Al.sup.3+, trivalent aluminum cations bind to the anionic groups of polymers to form a three-dimensionally cross-linked polymer, thus forming a gel. The gelation with Cr.sup.3+ is made by reducing Cr.sup.6+ to Cr.sup.3+ by a reducing agent, but the detailed gelation mechanism has not been sufficiently made clear. The correlation of gelation time for polymer solution is merely reported by Jordan and Ronald in Society of Petroleum Engineers Journal, April 229(1981).
Conventional gel-forming materials used for preventing the leakage of underground water have the disadvantage that the gel releases water at a high temperature, particularly in a high salinity brine system, thus resulting in collapse of the gel. For such a reason, no satisfactory effect has been obtained.
It is an object of the present invention to provide a water-soluble polymer suitable for use in preventing leakage of underground water, particularly underground water containing salts in high concentrations at high temperatures.
A further object of the present invention is to provide a composition capable of forming a strong and stable gel with an adequate gelation time even in a high salinity brine and high temperature system.
Another object of the present invention is to provide an improved process for preparing a strong and stable aqueous gel having a salt tolerance and a heat stability, which can be easily applied to improvement of water-oil ratio in oil production, prevention of leakage of underground water in civil engineering works and the like.
These and other objects of the present invention will become apparent from the description hereinafter.